1. Field of the Invention
The present invention relates to a polarizer which provides a very high extinction ratio (>1000), and more particularly, to a wire grid polarizer with double metal layers for use in visible spectrum and a fabrication method thereof.
2. Description of the Related Art
The use of an array of parallel conducting wires to polarize radio waves dates back more than 110 years. Wire grids, generally in the form of an array of thin parallel conductors supported by a transparent substrate, have also been used as polarizers for the electromagnetic spectrum.
FIG. 1 illustrates a conventional wire grid polarizer. The wire grid polarizer 100 comprises a multiplicity of parallel conductive electrodes 110 supported by a dielectric substrate 120. This device is characterized by the grating spacing or pitch or period of the conductors 110, designated as P; the width of the individual conductors 110, designated as W; and the thickness of the conductors 110, designated as D. A beam of light 130 produced by a light source 132 is incident on the polarizer at an angle θ from normal, with the plane of incidence orthogonal to the conductive electrodes 110. The wire grid polarizer 100 divides this beam 130 into a specularly reflected light beam 140 and a non-diffracted, transmitted light beam 150. The incident light 130 comprises TM and TE (Transverse Magnetic and Electric) polarized light. The TM polarized light is also referred to as a P polarized light and the TE polarized as an S polarized light. Using the normal definitions for the TM and TE polarized light; the TM polarized light has an electric field vector perpendicular to the wires 110 of the grid. Conversely, the TE polarized light has an electric field vector parallel to the wires 110 of the grid. In general, a wire grid polarizer will reflect the TE polarized light and transmit the TM polarized light. The polarization performance is determined by an extinction ratio of transmittance (i.e. TTM/TTE), wherein the symbol “TTM” is the transmittance of the TM polarized light and the symbol “TTE” is the transmittance of the TE polarized light. For simplicity, the extinction ratio of transmittance is referred to as the extinction ratio in this invention.
In U.S. Pat. No. 4,289,381, Garvin et al disclose a high selectivity thin film polarizer with double metal layers. The thin film polarizer is manufactured by traditional photolithography and etching processes. Nevertheless, the device is described relative to usage in the infrared spectrum (2˜100 μm), not the visible spectrum. That is, the reference does not teach how to design the polarizer with high quality extinction for the visible spectrum.
In U.S. Pat. No. 5,748,368, Tamada et al disclose a polarizer with single metal layer. The device has a specific ratio of wire length to width, grid spacing and trapezoidal wire shape. Nevertheless, the device provides a very low extinction ratio (about 30:1). Furthermore, the device only operates properly within narrow wavelength bands (about 25 nm) and the device is rather angularly sensitive. These considerations make the device unsuitable for broad wavelength bands in the visible spectrum.
In U.S. Pat. No. 6,122,103, Perkins et al disclose a broadband wire grid polarizer for the visible spectrum. The method changes the refractive index of the dielectric layer and etches slots into the substrate to form ribs, thereby increasing the polarization range of this device.
In U.S. patent application Ser. No. 2002/0122235, Kurtz et al disclose a wire grid polarizer. This device employs intra-wires of dielectric and metal to enhance the extinction ratio. The device is difficult to manufacture, however, as it requires at least six intra-wire layers.
In U.S. patent application Ser. No. 2002/0191286, Gale et al disclose a method of forming a polarizer with a continuous-relief profile. Nevertheless, the extinction ratio of this device is unstable. For example, the extinction ratio is about 20:1 when using a light wavelength of 550 nm.
Thus, there exists a need for an improved wire grid polarizer with very high extinction ratio (>1000), particularly for use in visible light systems. Moreover, there is a need for an improved wire grid polarizer for use at large incident angles, for example, for use in a LCD having a light source disposed at the lateral thereof.